This invention relates to optical fiber signal processors and more particularly to a ladder architecture for use in time and frequency domain signal processing applications such as filtering, matrix algebra operations, etc. The ladder architecture also relates to fiber optic sensors.
An optical fiber comprises a central core and a surrounding cladding. The refractive index of the core is greater than that of the cladding, and the diameter of the core is so small that light guided by the core impinges upon the core-cladding interface at an angle less than the critical angle for total internal reflection.
A light wave may be represented by a time-varying electromagnetic field comprising orthogonal electric and magnetic field vectors having a frequency equal to the frequency of the light wave. An electromagnetic wave propagating through a guiding structure can be described by a set of normal modes. The normal modes are the permissible distributions of the electric and magnetic fields within the guiding structure, for example, a fiber optic waveguide. The field distributions are directly related to the distribution of energy within the structure. The normal modes are generally represented by mathematical functions that describe the field components in the wave in terms of the frequency and spatial distribution in the guiding structure. The specific functions that describe the normal modes of a waveguide depend upon the geometry of the waveguide. For an optical fiber, where the guided wave is confined to a structure having a circular cross section of fixed dimensions, only fields having certain frequencies and spatial distributions will propagate without severe attenuation. The waves having field components that propagate unattenuated are the normal modes. A single mode fiber will propagate only one spatial distribution of energy, that is, one normal mode, for a signal of a given frequency.
A multimode fiber will propagate more than one normal mode of a given frequency. The number of guided modes in an optical fiber depends upon the diameter of the core.
Optical fibers are useful in signal processing systems because they provide greater rates of information transfer than are possible with wires carrying electrical signals. The speed of light in an optical fiber is greater than the speed of electrical signals in a conductor. Light signals in optical fibers also have the advantage of providing more communications channels than lower frequency electromagnetic waves.
Previous optical fiber architectures have been included as elements of sensor arrays. These prior architectures have employed symmetrical couplers and all single-mode optical fibers or all multimode optical fibers and symmetrical multimode couplers. See, for example, J. L. Brooks, Jr. et al., "Coherence Multiplexing of Fiber -Optic Interferometric Sensors," Lightwave Technology, Vol. LT-3, No. 5, pp. 1062-1072, Oct. 1985; A. R. Nelson, et al., "Passive Multiplexing System for Fiber-Optic Sensors," Applied Optics, V-19, N-17, pp. 2917-2920, Sept. 1980.
A systolic system includes a set of interconnected cells that are each capable of performing some simple operation. In a systolic system, the data flow is in a pipelined fashion. The data passes through many processing elements before leaving the system. Pipelining permits processing to proceed concurrently with input and output, thereby minimizing overall execution time. Systolic systems thus provide the advantages of effective use of data with high computation throughput, simple and regular data flows, use of simple and uniform cells and modular expandability.
Fiber optic architectures have been employed in fiber optic systolic matrix-vector multipliers. These architectures have employed recirculating loops, therefore, considerable time was required for recirculations or echoes from one input pulse sequence to die out before another input pulse sequence could be launched. See, for example, U.S. Pat. No. 4,588,255 "Fiber Optic Signal Processor for Matrix Vector Multiplication and Lattice Filtering" by Tur, Goodman, Moslehi, Bowers and Shaw. The disclosure of that patent is incorporated by reference into this disclosure. These recirculations limit the average duty cycle or total data throughput of a system.
Ladder structure may be used to eliminate the recirculation, but such structures lose a considerable portion of input light from the system due to the existence of free fiber ends at the output couplers. The loss of light reduces the amount of light that may be coupled to the output end of the output fiber.
Optical fibers can be made sensitive to a large number of physical phenomena, such as acoustic waves and temperature fluctations. An optical fiber exposed to such phenomena changes the amplitude, phase or polarization of light guide by the fiber. Optical fibers have been used as sensing elements in microphones, hydrophones, magnetometers, accelerometers and electric current sensors.
Mach-Zehnder, Michelson, Sagnac, and resonant ring interferometers have been used as sensors. Mach-Zehnder, Michelson and Sagnac interferometers respond to the phenomenon being sensed by producing phase differences in interfering light waves. Detecting phase changes in the waves permits quantitative measurements to be made on the physical quantity being monitored. The Sagnac interferometer produces phase differences in two counter-propagating light waves in a coil of a single fiber in response to rotations about the axis of the coil.
A fiber optic Mach-Zehnder interferometer typically has a reference arm comprising a first length of optical fiber and a sensing arm comprising a second length of optical fiber. The sensing arm is exposed to the physical parameter to be measured, such as an acoustic wavefront, while the reference arm is isolated from changes in the parameter. When the Mach-Zehnder interferometer is used as an acoustic sensor, acoustic wavefronts change the optical length of the sensing arm as a function of the acoustic wave pressure amplitude. An optical coupler divides a light signal between the two arms. The signals are recombined after they have propagated through the reference and sensing arms, and the phase difference of the signals is monitored. Since the signals in the reference and sensing arms had a definite phase relation when they were introduced into the arms, changes in the phase difference are indicative of changes in the physical parameter to which the sensing arm was exposed.